A process of branching morphogenesis underpins the formation of arborized epithelial networks in many organ systems including the lung and collecting duct system of the mammalian kidney. The collecting duct initiates on formation of the ureteric bud (UB) from the nephric duct. Cells at the bud tip, and at subsequent branch tips of the expanding ureteric epithelium, respond to inductive signals from the adjacent mesenchyme that promote and regulate branching morphogenesis, and maintain an uncommitted progenitor population at the branch tip. Progenitors exiting the tip to populate stalk regions differentiate ito the regionally distinct cell types of the collecting duct network. I aim to ascertain the unique molecular features of these tip cells to gain vital insights into stem cell dynamics and branching morphogenesis. I have utilized RNA sequencing data comparing gene expression in tip and stalk domains expression of the developing mouse kidney. These comparisons have identified a strong, tip-enriched set of genes that include all known tip regulatory genes from the literature. In Aim 1, I will perform a detailed in situ expression analysis of the tip- enriched gene set in th developing mouse kidney and lung to identify general and kidney specific regulators of epithelial morphogenesis. In Aim 2, I will develop a cell culture model of the ureteric tip population to systematically analyze the function of tip-enriched genes through genetic approaches. In Aim 3, I will explore the specific actions of Adamts18 (encoding a metalloproteinase with disintegrin-like and thrombospondin type 1 motif repeats) in the branching process. Adamts18 was identified as tip-enriched in both the kidney and lung. We have analyzed Adamts18 null mutants and shown that a subset of mutants display enlarged kidneys with double ureters. Thus, Adamts18 is normally required to restrict kidney branching and has emerged as a novel regulator of branching morphogenesis.